Only option (d) is correct ie. the speed is 1.0 m/s when the acceleration is half the initial value.
(B) The terminal speed is the constant speed when acceleration is zero. Thus
0 = 6.0-3v
v = 2 m/s
(C) At t=0, v=0; therefore initial acceleration a = 6.0-0 = 6.0 m/s2
(D) Acceleration is half that of initial value = 6.0/2 = 3.0m/s2
This gives 3 = 6-3v(t)
or v(t) = 1m/s